Estimating method of NOx occlusion amount

Abstract

The present invention is a method for estimating a NOx occlusion amount of a NOx occlusion catalyst interposed in an exhaust passage in an engine, which is constituted such that a NOx occlusion amount is estimated by using a polynomial reflected with NOx occlusion characteristics of a NOx occlusion catalyst and each coefficient of the polynomial is sequentially corrected on the basis of NOx purification rates actually measured.

What is claimed is:  
     
         1 . A method for estimating a NOx occlusion amount of a NOx occlusion catalyst interposed in an exhaust passage in an engine, characterized in comprising the steps of: 
 estimating said NOx occlusion amount using a polynomial reflected with a NOx occlusion characteristics of said NOx occlusion catalyst, and    correcting each coefficient of said polynomial sequentially on the basis of NOx purification rates actually measured.    
     
     
         2 . A method for estimating a NOx occlusion amount according to  claim 1 , characterized in that 
 the polynomial for obtaining the NOx occlusion amount x which is used in said estimating step includes a NOx purification rate r, an exhaust gas temperature y and an exhaust gas flow velocity z, and    said polynomial is a polynomial obtained by multiplying said exhaust gas temperature y and said exhaust gas flow velocity z by respective coefficients.    
     
     
         3 . A method for estimating a NOx occlusion amount according to  claim 2 , characterized in that said polynomial is expressed by the following equation;  
         x=[r− ( k   0   +k   2   y+k   3   z  . . . )]/( k   1   +k   4   y+ . . .  )  here, ki (i=1, 2, . . . ) are coefficients.    
     
     
         4 . A method for estimating a NOx occlusion amount according to  claim 2 , characterized in that said correcting step comprises, in an occasion of correcting said coefficient sequentially: 
 estimating the (N+1)-th NOx purification rate r on the basis of the N-th (N is a natural number) NOx occlusion amount x obtained from said polynomial, and    correcting each coefficient such that said estimated (N+1)-th NOx purification rate r becomes the NOx purification rate r actually measured.    
     
     
         5 . A method for estimating a NOx occlusion amount according to  claim 4 , characterized in that the coefficient is corrected by using the method of least square.  
     
     
         6 . A method for estimating a NOx occlusion amount according to  claim 1 , characterized in that a NOx discharging amount in said NOx occlusion catalyst is calculated according to the following equation.  
       NO x  discharging amount=∫ (reducing agent concentration at catalyst inlet×reducing agent utilization rate−0.5×oxygen concentration in catalyst inlet)×exhaust gas flow rate  
     
     
         7 . A method for estimating a NOx occlusion amount according to  claim 6 , characterized in that: 
 said reducing agent utilization rate is set on the basis of exhaust gas temperature y and exhaust gas flow velocity z, and at the same time    the characteristics of the reducing agent utilization rate are stored in a reducing agent utilization rate setting map.    
     
     
         8 . A method for estimating a NOx occlusion amount according to  claim 6 , characterized in that: 
 said reducing agent utilization rate is estimated using a polynomial which is reflected with a NOx discharging characteristics of the NOx occlusion catalyst, and    the coefficients of said polynomial are sequentially corrected on the basis of the concentration of reducing agent.    
     
     
         9 . A method for estimating a NOx occlusion amount according to  claim 8 , characterized in that: 
 the polynomial for obtaining the reducing agent utilization rate r′ includes a catalyst inlet reducing agent concentration x′, an exhaust gas temperature y and an exhaust gas flow velocity z, and    said polynomial is a polynomial obtained by multiplying said catalyst inlet reducing agent concentration x′, said exhaust gas temperature y and said exhaust gas flow velocity z by respective coefficients.    
     
     
         10 . A method for estimating a NOx occlusion amount according to  claim 9 , characterized in that the polynomial for obtaining the reducing agent utilization rate r′ is expressed by the following equation;  
       
         
           
             
               
                 
                   
                     
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       here, m i  (i=1, 2, . . . ) are coefficients.  
     
     
         11 . A method for estimating a NOx occlusion amount according to  claim 1 , is characterized in that: 
 said engine is constituted such that switching can be performed between a lean operation where an exhaust gas air-fuel ratio is lean and a rich operation where said exhaust gas air-fuel ratio is rich, and    said coefficients of the polynomial are held during said rich operation, and when a difference between the NOx purification rate obtained by using said held coefficients at a starting time of the lean operation and said NOx purification rate actually measured is equal to or more than a threshold value, said NOx occlusion amount is corrected.    
     
     
         12 . A method for estimating a NOx occlusion amount according to  claim 11 , characterized in that the NOx occlusion amount is corrected, when a difference between an actually measured value of the NOx purification rate r at the starting time of the lean operation of said engine and an estimated value thereof is equal to or more than a threshold value.  
     
     
         13 . A method for estimating a NOx occlusion amount according to  claim 12 , characterized in that said NOx occlusion amount is corrected based upon a judgment that a NOx occlusion amount calculated at the starting time of the lean operation is incorrect, when a difference between said NOx purification rate estimated by the polynomial and the NOx purification rate obtained by actual measurement immediately after switching is performed from the rich operation of said engine to the lean operation thereof is equal to or more than a predetermined value.  
     
     
         14 . A method for estimating a NOx occlusion amount according to  claim 1 , characterized in judging that said catalyst is abnormal, when an average value of said each coefficient in a predetermined period is deviated from a predetermined range.